The Secret Doctrine is the magnum opus of Helena Blavatsky. In it, Blavatsky lays out the fundamental basis for Theosophy, a religious philosophy that she believed to be an ageless, universal system for understanding how the earth came to be and humanity's connection to the unseen divine. She drew upon a number of disparate sources to craft her system, including Hindu reincarnation and Darwinian evolution. At its base, Theosophy teaches that everything—the human soul, the earth, the solar system—is proceeding through a set of seven reincarnation periods. The periods can be thought of as one infinitely long breath emanating from the Ultimate Truth. During the outbreath (expansion), everything cycles away from spiritual Oneness, culminating in the fourth period, when we are farthest away and thus visible in material form. After this apex, we begin the journey back, returning to the ultimate goal of all material forms: spiritual Oneness and Truth.